There are a lot of possiblities with the typings of Xerneas and Yveltal.
Imo, i think Yveltal would be dark flying and Xerneas would be ice psychic. It's pretty hard to guess Xerneas's typing lol. In the trailer when Xerneas takes a step, you see something like mist appear and when it zooms out there is mist around Xerneas, thats why i think Xerneas would be part ice. But it's strange cuz it does not fit the foresty background it's in. I think it would be psychic so it would be weak to dark(yveltal). Also, both of the leg have an advantage.
ice>flying
Dark>psychic

Another thing is that Yveltal could be part ghost. either ghost/flying or ghost/dark with levitate such as flygon (dragon/ground)
What leads me to believe that is because it has blue eyes which it zooms in on and it seems blind. In a norse legend the "bird/dragon" is supposed to be a ghost or something supernatural.What do u guys think?